When an optical wave is propagated in a medium, a portion of its energy can be converted to the kinetic energy of the medium by exciting acoustic phonons. This process can be facilitated by various mechanisms such as thermal or electrostriction effect. The excited acoustic phonons in turn generate inelastic scattering of the optical wave, known as Brillouin scattering. The magnitude and frequency (e.g., spectrum) of the Brillouin scattered light can be determined using the acoustic phonons generated therein. Such generated photons are likely closely related to mechanical properties of the medium, such as modulus, density, and structural shape. These mechanical properties therefore can be measured by examining the Brillouin scattered light. This technique is known as Brillouin spectroscopy. Various conventional techniques to detect the Brillouin signal have been applied in physics, material science, and mechanical engineering area. In addition, the Brillouin process can be enhanced via the use of multiple optical pump waves with frequencies separated by those of the acoustic phonons in the medium.
The Brillouin microscopy generally differs from the Raman microscopy or spectroscopy in that the Brillouin microscopy involves acoustic phonons instead of vibrational phonons Raman scattering is generally based upon. Since the Brillouin shift ranges typically from 10 MHz to 10 GHz, the direct electrical detection of the acoustic wave may also be possible.
Accordingly, there is a need to overcome the deficiencies described herein above.